It is well known that radio waves, propagating from a transmitter to a receiver, can follow a plurality of different paths, and that the relative phases of the different waves arriving at the receiving antenna can be such as to destructively interfere, causing what is commonly referred to as a fade. In order to reduce the opportunity for this to occur, the so-called "space diversity" system has been developed using two, spaced antennas to feed a common receiver. The theory underlying the use of two spaced-apart antennas is that there is less likelihood that a fade will occur at both antennas at the same time. In the simplest system, means are provided to disconnect the receiver from one antenna as soon as the received signal level falls below a predetermined threshold and to connect the receiver to the second antenna. In this so-called "blind switching," it is assumed that the signal received by the second antenna is stronger than that received by the first antenna. In a more sophisticated system, the signals from the two antennas are combined at radio frequency instead of switching between the two. This eliminates amplitude and phase jumps associated with the switching operation, and has the added advantage of delivering a larger amplitude signal to the receiver. However, such a system requires the use of dynamic phase correction to compensate for variations in the relative phase of the two signals caused by changes in the path lengths traversed by them. In one such system, described in U.S. Pat. No. 2,786,133, a single, continuously adjustable phase shifter is included in one of the antenna wavepaths and is automatically adjusted so that the wave from the one antenna has the proper phase to combine with the wave from the other antenna. U.S. Pat. No. 3,582,790 shows, in greater detail, a means for combining the two received signals and for isolating the two antennas from each other. The circuit includes a first phase shifter which shifts the phase of one of the input signals to bring it into quadrature relationship with the other. The quadrature related signals are combined in a first hybrid coupler to produce a pair of equal amplitude signals. The phase of one of the two signals is then shifted 90 degrees by a second phase shifter so as to bring the two signals in phase. The two equal, in-phase signals are then combined in a second hybrid coupler to produce a single output signal whose total power is equal to the sum of the powers of the two received signals.
Both of these systems seek to track the two signals continuously and do so by means of continuously variable phase shifters. The problem with such phase shifters is that in order to go from maximum phase shift back to zero, they must go through all phase values therebetween. To illustrate the problem, consider two waves whose relative phase difference is slowly increasing. As the phase increases, it will eventually reach 360 degrees at which point the two signals are again in phase. However, a phase shifter such as the type illustrated in U.S. Pat. No. 2,786,133 does not ease past its maximum phase shift to zero phase shift but, instead, must be reset by going completely through its entire range of phase shifts from its maximum setting to its minimum setting, causing a sudden fluctuation in the amplitude of the output signal, including the possibility of signal cancellation.
This return-toward-zero problem is avoided by using stepping phase shifters of the types disclosed in copending applications Ser. Nos. 578,528 and 878,561, filed concurrently on Feb. 17, 1978, wherein the signal phase can be advanced or retarded continuously in 90 degree steps. However, one limitation of this approach is that phase correction is made in discrete increments and, hence, is only approximate. For example, the two signals can be as much as 45 degrees out of phase, resulting in some signal loss due to phase error.
A second difficulty resides in the manner in which the phase shifter control signal is derived. Typically, a small phase modulation is impressed upon the signal in one of the two antenna circuits, as described, for example, in the article "Diversity Reception and Automatic Phase Correction" by L. Lewin, pp. 295-304, The Proceedings of The Institution of Electrical Engineers, July 1962. The phase modulation produces an amplitude modulation of the composite signal obtained when the two signals are combined. The fundamental and second harmonic of this amplitude modulation is then detected by the receiver's AGC circuit and used to control the phase shifter. The problem with this approach is that it is often difficult to accurately detect the relatively small second harmonic component in the presence of noise.